Heh, you're right, Emg, that certainly is a well-fed betta
I agree with Emg, he needs a serious fasting period. Why don't you get a net breeder, like this:
http://www.bigalsonline.com/catalog/product.xml?product_id=31683;category_id=2321
It hangs onto the side of your tank. Put your betta in there and he'll still be with the gang, but he won't be able to get at all the food. He'll probably get a little bit, but not near as much as he was.
Leave him in there for a few days until the lump goes down a lot, then feed him some skinned, cooked peas. I keep a bag of frozen in my freezer and once a week I take a small amount out and nuke them in the microwave with some water for about 45 seconds. Let them cool. Then *pop* the pea part out of the shell, squish 'em real good and put them in your tank. Feed a couple of peas to the whole gang -- I usually do 1 pea for 4-5 fish, depending on fish size -- and just give your betta a very *small* amount. Watch and make sure he eats it.
The peas will help him pass waste faster and easier. Once he gets down to his regular size, you can release him back into the tank.
But, you need to take a look at how you are feeding your fish. Bettas *are* pigs, like Emg says, but they are also slow. If you don't have a lot of top feeders and are feeding flakes, take the flakes and let them go *under* the water and into the filter outflow. The fish should have no problems chasing down the food and the betta will have to work hard to keep up. Great exercise and this should help with him hogging the food.
Also, it's possible that you are feeding too much. Cut back quite a bit. IMO once a day is fine if you are feeding twice or more.
Hope this helps,
Roan